Heretofore, a discharge lamp lighting device has been proposed, which includes an inverter circuit that converts direct current power into alternating current power and supplies the alternating current power to a discharge lamp (for example, refer to Patent Literature 1).
As this type of discharge lamp lighting device, for example, there is one shown in FIG. 15. This discharge lamp lighting device includes: a direct current power supply circuit 1 that converts a voltage of a direct current power supply E; an inverter circuit 2 that converts direct current power, which is outputted by the direct current power supply circuit 1, into alternating current power and supplies the alternating current power to a discharge lamp DL; and a control circuit 3 that controls a frequency of such an output of the inverter circuit 2. The discharge lamp DL is a so-called high pressure discharge lamp, and has an advantage that light distribution control thereof is easy since a relatively high luminous flux can be obtained with respect to a size thereof and the discharge lamp DL concerned can be therefore handled like a point source, but requires a high voltage pulse of several kilovolts for starting thereof.
The direct current power supply circuit 1 includes the direct current power supply circuit 1 composed of a well-known back converter including: a series circuit including a switching device Q2, an inductor L2, a capacitor C2 and a resistor R, which is connected between both terminals of the direct current power supply E; a diode D2 connected to a node between the resistor R and the direct current power supply E and a node between the switching device Q2 and the inductor L2; and a power supply control unit 1a that is made, for example, of a microcomputer and drives the switching device Q2. The direct current power supply circuit 1 includes voltage dividing resistors Ra and Rb connected between both terminals of the capacitor C2, and the power supply control unit 1a controls a frequency and a duty ratio for turning on/off the switching device Q2 based on a both-terminal voltage of the capacitor C2, which is divided by the voltage dividing resistors Ra and Rb.
Moreover, the inverter circuit 2 is an inverter circuit of a so-called full bridge type, which includes: two series circuits, each of which is composed of two switching devices among switching devices Q3 to Q6, and are connected in parallel to each other between output terminals of the direct current power supply circuit 1; and a series circuit including a parallel circuit composed of the discharge lamp DL and a capacitor Cr, and an inductor Lr, which are connected between a node between the switching devices Q3 and Q4 and a node between the switching devices Q5 and Q6.
The control circuit 3 drives the switching devices Q3 to Q6, which are located diagonally to each other, to turn on/off simultaneously, and drives the switching devices Q3 to Q6, which are connected in series to each other, to turn on/off alternately, thereby supplies the alternating current power to the discharge lamp DL.
As shown in FIG. 16, after the power supply is turned on, the conventional control circuit 3 operates, for a predetermined time, in a no-load mode of alternately repeating two periods in which, during one period, an on/off frequency of the switching devices Q3 to Q6 (hereinafter, referred to as an “operation frequency”) is set as high as approximately a resonance frequency of the inductor Lr and the capacitor Cr, and during the other period, the operation frequency is dropped more than the resonance frequency. Then, after the no-load mode is ended, the control circuit 3 proceeds to a lighting mode of dropping the operation frequency more than the resonance frequency. Specifically, during the period of raising the operation frequency, a discharge is started in the discharge lamp DL, and during the period of dropping the operation frequency, such a glow discharge started in the discharge lamp DL is shifted to an arc discharge, and the arc discharge is maintained in the lighting mode.
[Patent Literature 1]    Japanese Patent Laid-Open Publication No. 2004-265707